A variable displacement pump control as for an axial piston pump having a swash plate actuating piston generally provides for pressure compensation with or without auxiliary modulation to control the fluid pressure applied to the swash plate actuating piston to maintain a predetermined pressure drop across a variable area metering orifice embodied as in a directional control valve for a fluid motor. The following U.S. patents exemplify variable displacement pump controls including pressure compensation with or without auxiliary modulation; U.S. Pat. Nos.
3,512,178, May 12, 1970, Russell; PA1 3,726,093, Apr. 10, 1973, Malott; PA1 3,866,419, Feb. 18, 1975, Paul; PA1 3,941,513, Mar. 2, 1976, Malott; PA1 3,945,764, Mar. 23, 1976, Marietta.
Variable displacement pumps of the character indicated are often employed in conjunction with hydraulic systems including one or more directional control valves for controlling the operation of a corresponding number of fluid motors, and in existing controls, air may be introduced into the hydraulic system upon pump startup if there is air in the pump or in the intake line thereof. Moreover, in existing controls, the pump may be subject to pressure peaks or spikes of high intensity due to, for example, rapid closing of a directional control valve.
In the U.S. Pat. Nos. 3,941,513 and 3,945,764, a constant pressure pilot valve controls a modulator which bleeds off pressure acting on the swash plate actuating piston, thus to provide the pressure compensation feature and the modulator, in addition, has opposed areas exposed respectively to pressures upstream and downstream of a variable area orifice to maintain a predetermined pressure drop across said variable area orifice, whereby if the pressure drop exceeds the predetermined pressure drop, the modulator bleeds off the pressure acting on the swash plate actuating piston to effect decrease in pump displacement. In the event of malfunction of the modulator, such as sticking thereof, the pump may be overloaded even if a horsepower modulator is provided as contemplated in said U.S. Pat. Nos. 3,941,513 and 3,945,764.